Happy holidays from Fermilab

Working at the Technical Division during this holiday season wouldn't be quite right without its seasonal Julie Kurnat chalk drawing, which is on display in the common area in Trailer 157, directly behind the Industrial Center Building. Image: Julie Kurnat, TD

This is the final issue of Fermilab Today for 2013. Fermilab Today will begin publishing again on Thursday, Jan. 2, 2014. Happy holidays!

Physics in a Nutshell

Edgar Allan Poe and the beginning of time

Author Edgar Allan Poe, best known for his macabre poems and detective stories, proposed a solution to a paradox about the dark night sky.

If the universe is infinite and uniformly filled with stars, then any line of sight, when we look up into the sky, should eventually hit some distant star. If so, then the night sky would be as bright as the face of the sun, rather than dark. How can this be?

This paradox vexed 19th-century astronomers, but today the puzzle is solved. The universe is simply not old enough for light from such distant stars to reach our eyes. Wait a billion years, and another billion light-years of galaxies will help to fill the gaps.

It's not widely known that this solution was first proposed by Edgar Allan Poe, an author of horror and detective stories. In an essay called "Eureka: a Prose Poem" (1848), he speculates that stars are so distant that, for some, "no ray from it has yet been able to reach us at all," and he reminds the reader that light travels at a finite speed. He assumed implicitly that "the universe of stars" has a finite age, and together these are the three ingredients of the present-day explanation.

Poe's essay was more spiritual than scientific, his assumptions about the distances to most stars could not be tested by measurements of the day, and his implicit assumption that the universe of stars had a beginning was motivated by religious faith. This point about the beginning of time has always fascinated and frustrated humanity. Whether one starts by believing in a moment of creation or a universe that has always existed, one finds oneself asking either, "What happened before the beginning?" or "How did this uncreated world come to be?" It's hard to be comfortable with either scenario.

In the early 20th century, the discovery that the universe is expanding came as a shock because many scientists at the time expected it to be without beginning and largely unchanging. Using general relativity, which relates the rate of spatial expansion to the matter and energy that fill space, the current best measurements point back to an absolute beginning of time 13.8 billion years ago. It seems as though science has vindicated all of Edgar Allan Poe's beliefs.

However, extrapolating all the way down to a point of zero size, infinite temperature and infinite density goes beyond our present state of knowledge. Physicists are still learning how matter behaves at very high temperatures and energies, and the behavior of matter affects the expansion rate of space. The high energies currently being studied at the LHC, for instance, correspond to a temperature of quadrillions of degrees, which was the temperature of the universe a trillionth of a second after the naive Time Zero. If new phenomena appear at higher energies, then that first picosecond of history may need to be rewritten.

This distinction between the physics of the very early universe and the metaphysics of creation is something that I feel is important because it has led to misunderstandings. Physicists and journalists use the phrase "the big bang" in different ways: To physicists, it means the early expansion history and development of the universe as we know it, but in the popular press, it is often taken to mean creation itself. Using the physicist's definition, it's not wrong to ask, "What happened before the big bang?" In fact, this is an active area of scientific research.

Let it snow

Do we live in a 10-dimensional hologram?

From Slate, Dec. 16, 2013

The universe can seem bewildering at times. In the past century, we've learned an incredible amount about the cosmos: its 13.8 billion-year history, its structure (including the number and distribution of galaxies), and its possible future (increasingly rapid expansion forever). Yet two big mysteries still elude physicists: What happened to the universe in its first instants? And what is the connection between gravity and the other forces of nature?

Comment: Particle physics: together to the next frontier

From Nature, Dec. 18, 2013

As emerging players jostle old ambitions, Fermilab Director Nigel Lockyer calls for the next generation of particle physics projects to be coordinated on a global scale.

This year was a watershed for particle physics. The decades-long quest to discover the Higgs boson is essentially complete. Still abuzz after a Nobel Prize for the Higgs prediction, the particle-physics community is feeling satisfied. It is time to pause, reflect and consider what comes next.

Counting neutrinos, one flake at a time

This MINERvA event display shows a neutrino-electron scattering candidate event. We see nothing coming from the left, and then a single stopping particle traveling in the same direction as the beam. The colors indicate the amount of energy deposited at that point.

MINERvA is a neutrino scattering experiment that prides itself on being able measure in exquisite detail the probability that a neutrino will interact: We look for many different reactions on many different nuclei. However, in order to measure those probabilities, we have to know precisely how many neutrinos are produced in the first place.

Although Fermilab's Accelerator Division can tell MINERvA just how many protons it delivers to the target that starts the NuMI beamline, knowing how many neutrinos are actually produced after those protons hit the target is tricky. How many particles are made by the protons hitting the target? Where do those resulting particles go afterward? How many of them subsequently create neutrinos? How many of those neutrinos pass through the detector located a kilometer away?

Until now, MINERvA has had to rely on other experiments' measurements of what happens when protons hit a target to predict how many neutrinos pass through the detector. But now MINERvA has its own way to measure how many neutrinos were produced. Although we know little about neutrino interactions with atomic nuclei, there is one reaction that is extremely well known: when a neutrino interacts with an electron. By measuring that one process, we can predict how many neutrinos must have originally passed through the detector.

This is similar to predicting the number of snowflakes that fall on Fermilab's 27.5-square-kilometer campus by measuring the number of snowflakes that fall into a cup in the parking lot. We can't count all of the snowflakes that fall on the campus, but it's possible to count those in a small cup. By knowing the area covered by our cup, we can extrapolate to all of Fermilab grounds.

But here's the trick: You need to collect enough snow in the cup to make a precise measurement.

The probability of a neutrino scattering off an electron is precisely known, but it is tiny, even by the standards of rarely interacting neutrinos! So to measure this process you need a very fine-grained detector to see a single high-energy electron that seems to appear out of nowhere and that travels in the same direction as the neutrino beam (see above figure).

After analyzing hundreds of thousands of interactions, MINERvA found 121 of these events and predicts that only 33 of them are from background processes. In the new run, MINERvA expects to see at least 10 times more events and thus hopes to measure the number of neutrinos even more precisely.

—Debbie Harris, Fermilab, and Anne Norrick, College of William and Mary

Neutrino-electron scattering events form a peak around zero in the distribution of the electron energy (E) times the square of the angle (Θ) between the neutrino beam and the electron direction.

Jaewon Park, University of Rochester, will give a talk on this MINERvA result at today's wine and cheese seminar at 4 p.m.